The invention relates to an acceleration sensor for vehicles, having a liquid which is moved in a closed-off housing with an indicating medium of lower density and of a lower volume arranged in it and wherein a detector responds to a movement of the liquid.
A sensor of this type is known from German DE-OS 36 09 841 and British Patent 2,022,822. A gas, which is used as the indicating medium, is arranged in a housing which may be pyramidal or conical. In the quiescent condition, this gas is located above the liquid. Accelerations are determined by a transmitter receiver line which extends over the liquid level and in which the transmitter is arranged at the geodetically highest point of the housing. The receivers are located laterally at the housing and receive radiation which is reflected at the surface of the liquid or is refracted by it.
A sensor of this type presents multiple problems. Because of the relatively large mass of the moved liquid and the multiple interactions between it and the housing, the level is subject to constant completely nonuniform and unpredictable movements. As a result, the receivers, for short periods of time, are frequently already exposed to transmitting-power radiation because of the superposition of the multiple movements which, as a rule, are relatively undamped, although the acceleration to be detected is far below a critical limit value. A sensor of this type can therefore only be used to a limited extent. This limitation applies to the triggering of a vehicle occupant protection system, such as an airbag or a rollover bar, which may be irreversibly triggered only when an actual critical acceleration value is present.
For a sufficient effectiveness, the known sensor requires additional electronic devices which eliminate the mentioned disturbing influences. However, as a rule, this is possible only by filters and integrating elements. Both require certain time-related expenditures, with the result that a sensor of this type is not uncritical concerning its time-related ability to respond.
The invention is based on the object of providing a sensor of the initially mentioned type which has a simple construction and permits a reliable and fast recognition of an acceleration value to be detected.
The invention achieves this object by use of a level tube as the housing and by having the tube aligned relative to the acceleration to be determined. The detector responds to a defined position of the indicating medium within the liquid, when this indicating medium takes up a position indicating the acceleration value to be determined.
It is essential for the present invention that the indicating medium move within the fluid as a function of the acceleration. Vibrations, to the extent that they can occur in the liquid at all, have no influence on the movement and the position of the indicating medium, since the liquid compensates for these vibrations in itself. The position of the indicating medium therefore supplies a precise indication of the acceleration to be detected. This is particularly true when the difference in density between the liquid and the indicating medium is sufficiently large. The adjustment of the indicating medium is determined by the resulting acceleration of the liquid. This acceleration is composed of the gravitation and of the possibly existing dynamic acceleration components of the liquid in the direction determined by the alignment of the level and supplies the lift for the indicating medium.
The sensor may be constructed in a plurality of ways. Thus, it may be a conductivity sensor, which dips into an electrically conductive liquid. With respect to its conductivity, the indicating medium differs from that of the liquid and may, for example, have no conductivity. The sensor will then be arranged such that, at the acceleration value to be detected, the indicating medium is located at the location of the sensor. As a result, the value of the acceleration, in this case, is recognized by the jump in conductivity.
Another particularly advantageous construction development of the sensor consists of a transmitter receiver line for electromagnetic or acoustic waves which is aligned such that, at the acceleration value to be detected, the indicating medium is located at its transmission line. This results in a change of the damping of the transmitter receiver line. As a rule, the damping will then increase. This is caused by the double passage of the radiation through the boundary layer between the liquid and the indicating medium.
As a result of this simple construction, it is also possible to determine several differently directed acceleration components by the use of a corresponding number of such sensors. Here, each of the sensors is used for determining the relevant acceleration in the pertaining direction. By a corresponding alignment orientation of sensors, it is also possible to select a number of sensors which is lower than the number of the acceleration components to be determined.
A further development of the invention is possible by developing the housing for the sensor as a straight level tube which is inclined with respect to the acceleration to be determined. If, for example, an acceleration in the longitudinal direction of the vehicle is to be determined, the tube is inclined with respect to the longitudinal direction. In the quiescent position, or in the case of a no-acceleration movement of the vehicle, the indicating medium is located at the geodetically highest point of the tube, and in the case of an additional acceleration, is determined from the vectorial addition of the gravitation and the dynamic acceleration of the liquid which supplies the lift for the indicating medium.
By a sensor which is constructed in this manner, it is also possible to determine several acceleration components which extend vertically with respect to one another. For this purpose, the tubes are aligned in the direction of a common vertex. The transmitter receiver lines of the individual sensors extend in the direction of a common end point which, for the individual tubes, is also the zenith point of the vertex with respect to the plane in which the acceleration components ar located.
Other devices for improving the operation of the acceleration sensor is to have an additional moving-condition sensor which is connected independently of the acceleration sensor via an OR gate to trigger an occupant protection system when the motor vehicle is in a condition which approaches weightlessness. This connection can utilize a time delay wherein the occupant protection system is triggered when the vehicle is in the condition which approaches weightlessness, for longer than a set switching time (t). The moving-condition sensor can be a distance meter which determines the distance of the motor vehicle from a road surface, or it can respond to the vertical acceleration or a wheel rebound. The acceleration sensor and moving condition sensor can be housed in a compact constructional unit. The devices are also used for triggering an occupant protection system when the vehicle is in a condition which at least approaches weightlessness and which is therefore particularly critical.
In the case of weightlessness, the acceleration values in the vehicle are relatively low. Nevertheless it is necessary during a free fall, that the occupant protection system be triggered because, at the end of the weightless state, extremely high acceleration values will frequently occur (impact). The invention provides that the occupant protection system becomes operative as early as during the weightless state, in order to provide a sufficient protection of the vehicle occupants at the end of weightlessness.
Since occasionally a condition of the motor vehicle which approaches weightlessness is reached during the normal driving operation, a time delay feature on the weightlessness input is used for differentiating between such "normal" and "critical" situations. In order to avoid these types of situations which may occur when driving over a hump and in order to ensure that the occupant protection system is not triggered unnecessarily, the duration of the critical condition is determined by a time function element. If this weightlessness time duration is short the occupant protection system must not be triggered.
During the state of weightlessness a distance meter can determine a distance from the ground which is clearly above the normal value. During a state of weightlessness, the sensor for the vertical acceleration furnishes a value which is close to zero. Additionally, the rebound of one or several vehicle wheels, if it is equal to the maximally possible value, is also a good indication of the state of weightlessness.
Although a safety system for a rollover bar of a motor vehicle, which is controlled by a wheel rebound sensor, is known from DE-PS 35 45 874, in contrast to the invention, the state of the free fall is not taken into account because the signal of the wheel rebound sensor becomes operative only when, at the same time, an inclination switch responds which is actuated when a certain inclination is exceeded. However, in the case of a state of weightlessness, there is frequently no functional connection with the inclination of the motor vehicle. Accordingly, the instant invention provides a time delay for the weightlessness control and does not use an inclination input.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.